bug in pg2?

pg2:join/2 allows you to add the same Pid to the same group more than
once. Do we want to call this a bug or just an optimized implementation?
-Vance
Erlang (BEAM) emulator version 5.1.2 [source] [threads:0]
Eshell V5.1.2 (abort with ^G)
1> pg2:create(foo).
ok
2> pg2:get_members(foo).
[]
3> pg2:join(foo, self()).
ok
4> pg2:get_members(foo).
[<0.23.0>]
5> pg2:join(foo, self()).
ok
6> pg2:get_members(foo).
[<0.23.0>,<0.23.0>]